The present invention relates to a hydraulic control system for a continuously variable belt-drive automatic transmission for a motor vehicle, and more particularly to a system for controlling pressure of oil in a hydraulic circuit for the transmission.
A continuously variable transmission with a torque converter having a lockup clutch is known. The transmission is controlled by a high line pressure of 30 to 40 kg/cm.sup.2 at maximum and the torque converter is controlled by lower control pressure of 4 to 6 kg/cm.sup.2. The transmission is further provided with a multiple-disk friction clutch in a selector device and transfer clutch for a four-wheel drive system. These clutches are independently operated by respective pressures of oil. Accordingly, it is necessary to control the respective pressures with accuracy.
Japanese Patent Laid Open 57-161360 (U.S. Pat. No. 4,478,105) discloses a system in which the pressure of oil from an oil pump is adjusted by a line pressure control valve for controlling drive and driven pulleys of the transmission. The line pressure control valve produces a control pressure for controlling a torque converter, a lockup clutch, a lockup control valve, and a clutch for a selector device.
However, in such a system, since the oil pump must supply oil to various devices at different pressures, an oil pump having a large discharge capacity is required, as shown in FIG. 3b. Further, since efficiency of the oil pump reduces at high pressure as shown in FIG. 3a, pumping loss is large and the lifetime of the pump driving shaft is shortened.
On the other hand, when the transmission is rapidly upshifted, a large amount of oil is fed to a cylinder of a drive pulley. As a result, converter pressure reduces. Further, since it is difficult to adjust converter pressure, the control pressure for the lockup control valve and clutches becomes excessively large, and the control pressure may vary with the variation of the line pressure.
Accordingly, it is desirable for a hydraulic system for supplying control pressure for the torque converter to be separated from a hydraulic system for supplying the line pressure for the transmission so as to independently produce the necessary pressures.
Japanese Patent application Laid-open 59-17054 discloses a system in which two oil pumps are provided for controlling the drive pulley and the driven pulley of the transmission, respectively. However, it does not suggest a pump for supplying oil to a torque converter.